Orthopedic surgeons often use bone screws as a component of a stabilizing construct in a bone. Typically, the head of the bone screw will have a recessed socket into which a driver tool is inserted to screw the bone screw into or out of a patient's bone. There is often a need to adjust the insertion depth of the bone screw so that the head of the bone screw is at a desired height/position above the bone.
A conventional orthopedic bone screw has either a straight major and minor diameter or a combination of straight and tapered major and minor diameters. The flank spacing, major diameter and minor diameter may vary between types of orthopedic screws.
When a conventional orthopedic screw is inserted into a bone, bone is displaced. One problem with a conventional tapered orthopedic bone screw is that it is narrowest at the insertion point and tapers to a larger diameter at the head. As the screw is inserted into the bone, the tapered thread displaces more bone along its axis as the screw is driven into place. This results in a snug fit as long as the screw remains in its initial position. If the screw is reversed, which causes the screw thread to back out, there is a gap between the screw form and the prior displaced bone, resulting in an instable construct.
A further related problem known in the art is that during subsequent orthopedic surgeries, known in the art as revision or repair surgeries, a bone screw often must be removed (e.g., as a result of loosening) and replaced with a larger screw. Because it is a general practice to initially use the largest diameter screw possible, a larger diameter screw cannot be used to the replace the initial screw. Removing the initial screw without a replacement results in a less structurally sound construct.
Bone loss due to successive insertion of orthopedic screws is undesirable, particularly with respect to osteoporotic bone, i.e., bones that are weakened by a decrease in bone density making them more susceptible to fractures.
Bone screws known in the prior art typically have minor and major diameters that are tapered so that the diameter increases from the tip of the screw to the head which increasingly compresses the bone as the bone screw is screwed deeper into the bone, resulting in a tight fit. These bone screws are tapered not only to provide an increasing compressive fit as the screw is screwed into the bone, but also to increase the loading force on the screw near the screw head. The thicker screw portion near the head makes the screw strongest where the lateral load on the screw is the greatest. An example of a tapered bone screw is disclosed by U.S. Pat. No. 5,226,766 (Lasner '766).
Tapered bone screws, such as the one disclosed by Lasner '766, however, are not desirable. Bone screws are available in various lengths and diameters. For stability and maximum anchoring ability, an orthopedic surgeon will use the largest possible diameter bone screw, i.e., the largest bone screw that will not breech the pedicle wall. During revision/repair surgery, a larger diameter screw may be used to provide secure retention of the repair bone screw in a hole previously used by a then-removed bone screw. If the larger diameter screw causes the pedicle wall to be breeched, the patient may suffer nerve damage. A tapered bone screw also exerts a radial force on the bone which may cause the bone to fracture.
In addition, if a tapered bone screw is ever backed out of the bone for adjustment or replacement of the screw, the replaced bone screw becomes loosened and will not be securely held by the bone due to the mating of the tapered bone screw with the tapered bore in the bone.
It is desirable to have an orthopedic screw which minimizes bones loss and the potential for fracturing, particularity for osteoporotic bone.
It is desirable to have an orthopedic screw which does not create instability during repositioning of the screw, if necessary, as repositioning often results in the creation of a larger hole to screw ratio.